As an ion exchange membrane to be used in an alkali chloride electrolysis method for producing an alkali hydroxide and chlorine by electrolyzing an aqueous alkali chloride solution such as saline, an electrolyte membrane made of a fluorinated polymer having ion exchange groups (such as carboxylic acid groups or carboxylate groups, sulfonic acid groups or sulfonate groups) is known.
As such an ion exchange membrane, for example, Patent Document 1 discloses an ion exchange membrane for alkali chloride electrolysis which comprises a layer (C) comprising a fluorinated polymer having carboxylic acid functional groups, a layer (S) comprising a fluorinated polymer having sulfonic acid functional groups, and a reinforcing material, wherein the respective ion exchange capacities of a layer (Sa) and a layer (Sb) contained in the layer (S) are in a predetermined relationship (Claim 1, etc.).